Yan, and 4 co-authors, reported in November, 2022 that the history of tropospheric O3, an important atmospheric oxidant, is poorly constrained because of uncertainties in its historical budget and a dearth of independent records. Here, The authors estimated the mean tropospheric O3 burden during the Last Interglacial period (LIG; 115 to 130 thousand years ago) using a record of the clumped isotopic composition of O2 (i.e., Δ36 values) preserved in Antarctic ice. The measured LIG Δ36 value is 0.03 ± 0.02‰ (95% CI) higher than the late pre-industrial Holocene (PI; 1,590–1,850 CE) value and corresponds to a modeled 9% reduction in LIG tropospheric O3 burden (95% CI: 3%–15%), caused in part by a substantial reduction in biomass burning emissions during the LIG relative to the PI.
(A) shows reconstructions of delta 36 (i.e., 18O -18O) in Antarctic ice for the Penultimate
Glacial Maximum (PGM) and the Last Interglacial (LIG) ( Yan et al. [2022], Figure 1a) and
(B) shows the probability distributions of those data as well as data generated by prior
studies of the present day (PD), pre-industrial (PI) and the Last Glacial Maximum (LGM)
( Banerjee et al. [2022]; Yeung et al. [2019]).
These results are consistent with the hypothesis that late-Pleistocene megafaunal extinctions caused woody and grassy fuels to accumulate on land, leading to enhanced biomass burning in the preindustrial Holocene.
In Plain Language: When humans first began impacting the Earth's atmospheric chemistry is unclear because its “natural” state just prior to human intervention is not precisely known. Here, the athors reconstructed the history of an important atmospheric oxidant—tropospheric ozone or O3—during the Last Interglacial period ∼127,000 years ago when Earth's climate was similar to today but humans had not yet dispersed widely across the continents. The authors found that the Last Interglacial tropospheric ozone levels were lower than during the late 17th to 19th centuries by about 10%, most likely due to differing amounts of biomass burning. This result can be explained by a global increase in biomass burning after megafaunal grazers went extinct and caused woody and grassy fuels to accumulate on land.
The authors concluded that clumped isotopes of O2 (Δ36) in air trapped in ice cores, which reflect global-scale trends in O3 photochemistry (Yeung et al., 2016, and 2021), provide distinct information about the tropospheric O3 burden during the Last Inter-glacial. The inferred 9% reduction (95% CI: 3%–15%) of tropospheric O3 in the LIG compared to the late Holocene was likely due to a shift in the global fire regime, though other factors might also contribute. This result is consistent with the hypothesis that widespread late-Pleistocene extinctions of herbivorous megafauna, potentially related to/catalyzed by the presence of humans, led to an accumulation of grassy and woody fuels (Johnson, 2009; Rule et al., 2012). A Δ36 record spanning the PGM through the Holocene may elucidate when this global-scale shift in atmospheric oxidation chemistry began.
Entry summary: https://eos.org/editor-highlights/clumped-18o-18o-in-ice-reveals-past-ozone-and-wildfire
Original working URL: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2022GL101113
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